Generally data is communicated using radio signals by modulating the data onto the radio signals in some way, and transmitting the radio signals to a receiver. At the receiver, the radio signals are detected and the data recovered from the received radio signals. Typically this is performed digitally, so that at the receiver, the detected radio signals are down converted to a base band representation and converted from analogue form to digital form. In the digital form the base band signals are processed to recover the data. However in order to recover the data, the receiver must be synchronised to the received digital signal samples to the effect that the relative temporal position of the recovered data symbols corresponds with the temporal position of the data when transmitted. This is particularly, but not exclusively true for radio communications systems in which the data is transmitted as bursts or packets of data.
An example of a radio communications system in which data is communicated in bursts or blocks of data is the Digital Video Broadcasting (DVB) system. The DVB system utilises a modulation scheme known as Coded Orthogonal Frequency Division Multiplexing (COFDM) which can be generally described as providing K narrow band carriers (where K is an integer) and modulating the data in parallel, each carrier communicating a Quadrature Amplitude Modulated (QAM) symbol. Since the data is communicated in parallel on the carriers, the same symbol may be communicated on each carrier for an extended period. Generally, this period is arranged to be greater than a coherence time of the radio channel. By averaging over the extended period, the data symbol modulated onto each carrier may be recovered in spite of time and frequency selective fading effects which typically occur on radio channels.
To facilitate detection and recovery of the data at the receiver, the QAM data symbols are modulated onto each of the parallel carriers contemporaneously, so that in combination the modulated carriers form a COFDM symbol. The COFDM symbol therefore comprises a plurality of carriers each of which has been modulated contemporaneously with different QAM data symbols.
In the time domain, each COFDM symbol is separated by a guard period which is formed by repeating data bearing samples of the COFDM symbol. Therefore, at a receiver, to detect and recover the data, the receiver should be synchronised to each COFDM symbol. The data is recovered from the COFDM symbol by performing a Discrete or Fast Fourier Transform on the data bearing signal samples of the COFDM symbol. It is therefore necessary to identify a symbol synch time after which for example, the signal samples are assumed to correspond to the data bearing signal samples of the COFDM symbol.
A previously proposed technique for acquiring synchronisation with the data bearing signal samples of a COFDM symbol is to cross correlate two samples which are temporally separated by the period over which the data bearing samples are modulated. A relative temporal position of the two samples is then shifted within the COFDM symbol, until a position is found at which the cross-correlation produces maximum energy.
Although the previously proposed synchronisation technique works adequately in the presence of additive white gaussian noise, in some situations such as where the signal is received in the presence of multi-path propagation, this technique produces a sub-optimum synchronisation point. A sub-optimum synchronisation point can cause the data bearing signal samples to be corrupted with energy from adjacent signal samples. This is known as inter-symbol interference (ISI).